Stereoselectivity metal-mediated reactions

Regio- and stereoselective metal-mediated l,3-butadien-2-ylation reactions between 1,4-dibromo-2-butyne or l,4-bis(methanesulfonyl)-2-butyne and optically pure azetidine-2,3-diones in aqueous media offers a convenient asymmetric entry to potentially bioactive 3-substituted-3-hydroxy-p-lactams 19 <02JOC1925>. In addition, 2-azetidinone-tethered 1,3-butadienes can easily be transformed into other functionalities via Diels-Alder reaction. 

One of the most versatile methodologies of EPC synthesis of allenes is the chirality transfer reaction which involves highly stereoselective, mechanism-controlled, metal-mediated propyn-yl-allenyl transpositions. Of these, the organocopper(l)-mediated reactions are especially useful and provide a relatively straightforward route to a broad variety of allenes of high enantiomeric purity. 

Stereoselective oxonium ylide reaction, in particular the asymmetric catalysis, has been a problem of considerable challenge in this field. Since the first report by McKervey and co-workers in the asymmetric induction in metal carbene-mediated ylide formation/[2,3]-sigmatropic rearrangement in 1992, there have been efforts being directed... 

A Lewis basic carbonyl group can be activated by coordination with a metal-centered Lewis acid, with profound reactivity and stereochemical consequences. In the context of asymmetric synthesis many Lewis acid-mediated reactions are known to proceed with better stereoselectivity than their non-catalyzed counterparts—very recently a variety of chiral Lewis acids have been shown to be remarkably efficient... 

Chiral allylic indium reagents and achiral aldehydes. Chiral indium reagents have been generated by the oxidative metallation of allylic bromides, which bear remote stereogenic subunits. For example, the 1,4-asymmetric induction in the indium-mediated coupling of allyl bromides 298 with benzaldehyde affords the syn-adducts as the major products (Table 10-35) [199]. The stereoselectivity of the reaction improves with more sterically encumbered allyl bromides. 

Triorganylbismuthines can mediate such C-C bond forming reactions as shown in Scheme 5.38 in the presence or absence of a transition metal salt. Tributylbismuthine promotes the allylation of carbonyl compounds with allylic bromide [91HAC297] and the olefination of diazo compounds with carbonyl compounds in the presence of catalytic amounts of a copper(I) halide [90TL5897]. A combination of triphenylbismuthine and WCle promotes the metathesis of olefins, while a similar combination with TiC facilitates the stereoselective Diels-Alder reaction of unsaturated esters [76TMC183, 93JOC4783]. 

Scheme 8.3. Some examples of V+5-mediated reactions of allylic alcohols with r-BuOOH. (a) A chemoselective reaction [8]. (b) Stereoselective reactions of acyclic allylic alcohols, compared to results obtained using m-CPBA [9]. Note that better selectivity is usually obtained using the metal-based oxidation system, but not always with the same relative topicity as observed using a peracid.

Similar studies involving metal-mediated carbene polymerization using diazocarbonyl monomers were reported in 2006 by de Brain. These showed that rhodium-based catalysts could be used for the stereoselective polymerization of carbenes generated from alkyl diazoacetates (Figure 31.5) [26]. Indeed, such catalysts were the first to produce high-molecular-weight, functionalized polymethylene 3, and demonstrated the first use of a mononuclear Rh(I) species (4 and 5) for carbene transfer reactions. The resulting poly(alkyl 2-ylideneacetate)s displayed... 

The Li group described a potassium-tert-butoxide-mediated stereoselective addition of quinazolines to terminal alkynes, employing mild, transi-tion-metal-free reaction conditions, leading to the formation of the T-selective addition product in moderate-to-good yields (Scheme 47) (13OBC5908). 

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